At least 250,000 people end up in hospital every year because of the damaging side- effects of the medicines they are taking and about 5,000 die, according to the British Medical Association.

The BMA urged doctors yesterday to be more vigilant and report any suspected side-effects their patients might experience. Only an estimated 10% of adverse drug reactions are currently reported through the "yellow card" scheme to the drug regulator, the Medicines and Healthcare products Regulatory Authority (MHRA).

But alerting the watchdog to potential problems with a herbal or conventional medicine can save lives, the BMA said. In 2003, after reports that some people taking kava kava for stress relief and insomnia suffered liver damage, the herbal medicine was withdrawn from sale in the UK. In 2000 the drug cisapride, for gastric trouble was banned, because reports showed it caused serious heart problems.

"Doctors have a professional duty to report all adverse drug reactions, especially if children or the elderly are involved," said Vivienne Nathanson, head of ethics and science at the BMA.

"Unfortunately too many health professionals are confused about reporting procedures. Doctors must make sure they report any suspected [adverse drug reactions] and at the same time increase awareness among their patients about the reporting process."

There could never be a guarantee that a medicine is completely safe, said a BMA guide for healthcare professionals entitled Reporting Adverse Drug Reactions which was launched yesterday.

Any drug that has a beneficial effect on the body may also have unwanted effects. A study published in 2004 found 6.5% of people admitted to hospital had suffered an adverse drug reaction, and in 80% of those, the drug was the direct cause of the admission.

On average, the patients spent eight days in hospital. More than 2% of those admitted in this way died.

That research revealed that the damaging side-effects of drugs were responsible for 4% of hospital bed capacity and cost the NHS £466m a year.

Not all side-effects would be picked up in clinical trials before the medicine got its licence, said Sir Charles George, chairman of the BMA's board of science. Drugs are usually tested on no more than 2,000 to 3,000 people. "If you have a very rare effect which happens once in 10,000 people, there is absolutely no likelihood that will be recognised prior to marketing," he said.

That meant greater vigilance was needed on the part of doctors, pharmacists and nurses - who can all prescribe medicines - and patients themselves, who have been allowed to report side-effects since January.

The MHRA urged healthcare professionals to use the yellow card scheme. "There is no need to prove that the medicine caused the adverse reaction, just the suspicion is good enough," said June Raine of the agency, which has provided posters for doctors to hang in their surgeries encouraging patients to report too. New drugs are marked in the British National Formulary - the drug prescribers' bible - with a black triangle. It was important that all suspected adverse side-effects of such medicines were reported, the BMA's guide said.

It was also very important that all side effects in children were reported. Drug trials are often not done on children, which means that doctors have to estimate the appropriate dose of an adult medicine when treating them. But children were not small adults, said Dr Nathanson, and could sometimes react in a different way.

It has long been known that doctors do not report all the suspected side-effects their patients tell them about. Ten years ago, the BMA issued similar guidance to doctors, but it had little effect.

The number of reports received each year by the MHRA has remained fairly constant, at around 20,000 since the mid-1980s. Various reasons, from having too much to do, to not having a supply of yellow cards to fill in, to lethargy, have been put forward.